restructure: move bin/ and tests/ to src/

Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.

Motivations:

  * Scripts and tests no longer have to do dubious things like:

        require('ep_etherpad-lite/node_modules/foo')

    to access packages installed as dependencies in
    `src/package.json`.

  * Plugins can access the backend test helper library in a non-hacky
    way:

        require('ep_etherpad-lite/tests/backend/common')

  * We can delete the top-level `package.json` without breaking our
    ability to lint the files in `bin/` and `tests/`.

    Deleting the top-level `package.json` has downsides: It will cause
    `npm` to print warnings whenever plugins are installed, npm will
    no longer be able to enforce a plugin's peer dependency on
    ep_etherpad-lite, and npm will keep deleting the
    `node_modules/ep_etherpad-lite` symlink that points to `../src`.

    But there are significant upsides to deleting the top-level
    `package.json`: It will drastically speed up plugin installation
    because `npm` doesn't have to recursively walk the dependencies in
    `src/package.json`. Also, deleting the top-level `package.json`
    avoids npm's horrible dependency hoisting behavior (where it moves
    stuff from `src/node_modules/` to the top-level `node_modules/`
    directory). Dependency hoisting causes numerous mysterious
    problems such as silent failures in `npm outdated` and `npm
    update`. Dependency hoisting also breaks plugins that do:

        require('ep_etherpad-lite/node_modules/foo')

src/tests/frontend/specs/drag_and_drop.js

@@ -0,0 +1,171 @@
+'use strict';
+
+// WARNING: drag and drop is only simulated on these tests, manual testing might also be necessary
+describe('drag and drop', function () {
+  before(function (done) {
+    helper.newPad(() => {
+      createScriptWithSeveralLines(done);
+    });
+    this.timeout(60000);
+  });
+
+  context('when user drags part of one line and drops it far form its original place', function () {
+    before(function (done) {
+      selectPartOfSourceLine();
+      dragSelectedTextAndDropItIntoMiddleOfLine(TARGET_LINE);
+
+      // make sure DnD was correctly simulated
+      helper.waitFor(() => {
+        const $targetLine = getLine(TARGET_LINE);
+        const sourceWasMovedToTarget = $targetLine.text() === 'Target line [line 1]';
+        return sourceWasMovedToTarget;
+      }).done(done);
+    });
+
+    context('and user triggers UNDO', function () {
+      before(function () {
+        const $undoButton = helper.padChrome$('.buttonicon-undo');
+        $undoButton.click();
+      });
+
+      it('moves text back to its original place', function (done) {
+        // test text was removed from drop target
+        const $targetLine = getLine(TARGET_LINE);
+        expect($targetLine.text()).to.be('Target line []');
+
+        // test text was added back to original place
+        const $firstSourceLine = getLine(FIRST_SOURCE_LINE);
+        const $lastSourceLine = getLine(FIRST_SOURCE_LINE + 1);
+        expect($firstSourceLine.text()).to.be('Source line 1.');
+        expect($lastSourceLine.text()).to.be('Source line 2.');
+
+        done();
+      });
+    });
+  });
+
+  context('when user drags some lines far form its original place', function () {
+    before(function (done) {
+      selectMultipleSourceLines();
+      dragSelectedTextAndDropItIntoMiddleOfLine(TARGET_LINE);
+
+      // make sure DnD was correctly simulated
+      helper.waitFor(() => {
+        const $lineAfterTarget = getLine(TARGET_LINE + 1);
+        const sourceWasMovedToTarget = $lineAfterTarget.text() !== '...';
+        return sourceWasMovedToTarget;
+      }).done(done);
+    });
+
+    context('and user triggers UNDO', function () {
+      before(function () {
+        const $undoButton = helper.padChrome$('.buttonicon-undo');
+        $undoButton.click();
+      });
+
+      it('moves text back to its original place', function (done) {
+        // test text was removed from drop target
+        const $targetLine = getLine(TARGET_LINE);
+        expect($targetLine.text()).to.be('Target line []');
+
+        // test text was added back to original place
+        const $firstSourceLine = getLine(FIRST_SOURCE_LINE);
+        const $lastSourceLine = getLine(FIRST_SOURCE_LINE + 1);
+        expect($firstSourceLine.text()).to.be('Source line 1.');
+        expect($lastSourceLine.text()).to.be('Source line 2.');
+
+        done();
+      });
+    });
+  });
+
+  /* ********************* Helper functions/constants ********************* */
+  const TARGET_LINE = 2;
+  const FIRST_SOURCE_LINE = 5;
+
+  const getLine = (lineNumber) => {
+    const $lines = helper.padInner$('div');
+    return $lines.slice(lineNumber, lineNumber + 1);
+  };
+
+  const createScriptWithSeveralLines = (done) => {
+    // create some lines to be used on the tests
+    const $firstLine = helper.padInner$('div').first();
+    $firstLine.html('...<br>...<br>Target line []<br>...<br>...<br>' +
+        'Source line 1.<br>Source line 2.<br>');
+
+    // wait for lines to be split
+    helper.waitFor(() => {
+      const $lastSourceLine = getLine(FIRST_SOURCE_LINE + 1);
+      return $lastSourceLine.text() === 'Source line 2.';
+    }).done(done);
+  };
+
+  const selectPartOfSourceLine = () => {
+    const $sourceLine = getLine(FIRST_SOURCE_LINE);
+
+    // select 'line 1' from 'Source line 1.'
+    const start = 'Source '.length;
+    const end = start + 'line 1'.length;
+    helper.selectLines($sourceLine, $sourceLine, start, end);
+  };
+  const selectMultipleSourceLines = () => {
+    const $firstSourceLine = getLine(FIRST_SOURCE_LINE);
+    const $lastSourceLine = getLine(FIRST_SOURCE_LINE + 1);
+
+    helper.selectLines($firstSourceLine, $lastSourceLine);
+  };
+
+  const dragSelectedTextAndDropItIntoMiddleOfLine = (targetLineNumber) => {
+    // dragstart: start dragging content
+    triggerEvent('dragstart');
+
+    // drop: get HTML data from selected text
+    const draggedHtml = getHtmlFromSelectedText();
+    triggerEvent('drop');
+
+    // dragend: remove original content + insert HTML data into target
+    moveSelectionIntoTarget(draggedHtml, targetLineNumber);
+    triggerEvent('dragend');
+  };
+
+  const getHtmlFromSelectedText = () => {
+    const innerDocument = helper.padInner$.document;
+
+    const range = innerDocument.getSelection().getRangeAt(0);
+    const clonedSelection = range.cloneContents();
+    const span = innerDocument.createElement('span');
+    span.id = 'buffer';
+    span.appendChild(clonedSelection);
+    const draggedHtml = span.outerHTML;
+
+    return draggedHtml;
+  };
+
+  const triggerEvent = (eventName) => {
+    const event = new helper.padInner$.Event(eventName);
+    helper.padInner$('#innerdocbody').trigger(event);
+  };
+
+  const moveSelectionIntoTarget = (draggedHtml, targetLineNumber) => {
+    const innerDocument = helper.padInner$.document;
+
+    // delete original content
+    innerDocument.execCommand('delete');
+
+    // set position to insert content on target line
+    const $target = getLine(targetLineNumber);
+    $target.sendkeys('{selectall}{rightarrow}{leftarrow}');
+
+    // Insert content.
+    // Based on http://stackoverflow.com/a/6691294, to be IE-compatible
+    const range = innerDocument.getSelection().getRangeAt(0);
+    const frag = innerDocument.createDocumentFragment();
+    const el = innerDocument.createElement('div');
+    el.innerHTML = draggedHtml;
+    while (el.firstChild) {
+      frag.appendChild(el.firstChild);
+    }
+    range.insertNode(frag);
+  };
+});